Microsoft has recently identified a significant issue affecting users of the new Outlook for Windows, where Excel attachments fail to open if their filenames contain non-ASCII characters. This problem, tracked under the reference ID EX1189359, results in an error message advising users to Try opening the file again later, leaving many confused about the cause of the blockage.
The issue specifically targets the modern new Outlook client and does not impact the classic version of the software. According to Microsoft’s service health dashboard, the root cause lies in an encoding error within the request used to open the files. When an Excel file attached to an email includes non-standard characters such as accented letters, symbols, or non-English scripts, the application fails to process the filename correctly, resulting in an immediate error.
This issue has been ongoing since late November 2025, with initial reports surfacing around November 23. While the scope is limited to specific file naming conventions, the impact is significant for international users or organizations that frequently use non-ASCII characters in document titles.
Microsoft’s Response and Fix Status
As of December 1, 2025, Microsoft engineers have developed a fix to address the missing encoding in file-handling requests. However, the solution is not yet available to all users. The company is currently in the validation phase, testing the deployment to ensure it resolves the error without introducing secondary issues. Microsoft is also investigating why this encoding error occurred to prevent similar regressions in future updates. A further status update is expected by the evening of December 1, UTC time.
Temporary Workarounds
Until the patch is fully rolled out, Microsoft has provided two official workarounds for users who need immediate access to their spreadsheets:
– Use Outlook on the Web (OWA): The web-based version of Outlook correctly processes these attachments, bypassing the client-side encoding failure.
– Download the File Locally: Users can save the attachment to their computer first. Once the file is saved to a local drive, it can be opened directly in Excel without triggering the Outlook previewer error.
This incident adds to the challenges faced by the new Outlook client, which has been scrutinized for feature-parity gaps compared to the classic COM-based application. Administrators can track the progress of the fix in the Microsoft 365 admin center under EX1189359.
Broader Context: Recent Microsoft Outlook Vulnerabilities
This recent bug is part of a series of issues and vulnerabilities that have affected Microsoft Outlook in 2025. Earlier this year, several critical vulnerabilities were identified and addressed:
– CVE-2025-47176: Disclosed on June 10, 2025, this vulnerability allows remote code execution (RCE) under certain local exploitation conditions. With a CVSS v3.1 score of 7.8, it poses a significant risk to both enterprise and individual users. The flaw involves a path traversal issue that could enable attackers to execute malicious code without user interaction once initiated. Microsoft has released patches to address this vulnerability, and users are strongly advised to update their systems promptly.
– CVE-2025-21298: Reported in January 2025, this critical vulnerability in the Windows Object Linking and Embedding (OLE) technology allows remote code execution. Rated 9.8 out of 10 on the Common Vulnerabilities and Exposures scale, it can be exploited via malicious rich text format documents, often sent as attachments or links through phishing campaigns. Microsoft has released updates to mitigate this risk, and users are urged to apply these patches immediately.
– CVE-2025-32705: Addressed during the May 2025 Patch Tuesday, this RCE vulnerability in Microsoft Outlook stems from improper memory handling. An attacker could exploit this flaw by sending a specially crafted file to the target user, leading to arbitrary code execution on the local system. Microsoft has provided fixes for this vulnerability, and users should ensure their systems are updated accordingly.
Recommendations for Users
Given the recent issues and vulnerabilities affecting Microsoft Outlook, users are advised to take the following steps to ensure their systems remain secure and functional:
1. Apply Updates Promptly: Regularly check for and install updates provided by Microsoft to address known vulnerabilities and bugs.
2. Use Web-Based Alternatives: In cases where the desktop client is affected by issues, consider using Outlook on the Web (OWA) as a temporary solution.
3. Be Cautious with Attachments: Avoid opening attachments from unknown or untrusted sources, especially those with non-standard characters in their filenames.
4. Monitor Official Channels: Stay informed about known issues and their resolutions by monitoring Microsoft’s official communications and service health dashboards.
By staying vigilant and proactive, users can mitigate the risks associated with these vulnerabilities and ensure a more secure and efficient email experience.
